1. The Field of the Invention
The present invention generally relates to sensors. More particularly, embodiments of the invention relate to vibrating wire gauges.
2. The Relevant Technology
Vibrating wire gauges are widely used in a variety of applications, including measuring the magnitude of various properties and forces in many construction projects, including buildings, bridges, dams, piles, tunnel linings, pipelines, anchors, and others. The gauges have been adapted to monitor stress, strain, deflection, pressure, displacement, fluid level, angular motion, and temperature. Although advancing technology has produced other types of sensors, the vibrating wire gauge is often considered the best sensor for use in many settings, due to the sensor's long-term reliability.
The vibrating wire gauge generally operates on the vibrating wire principle which states that a wire vibrates at its resonant frequency when plucked. The resonant frequency is determined by
      v    =                  n                  2          ⁢          l                    ⁢                        σ          μ                      ,where v is frequency in cycles/time, n is 1 cycles for the fundamental (non-harmonic) vibration mode, l is the length of the wire, σ is the wire tension (or stress) in force/area, and μ is the wire's length density in mass/length. The gauge is constructed so that a wire is held in tension inside a small diameter, thin-walled tube that is welded or otherwise attached to a structural member. An electromagnetic coil is used to pluck or excite the wire and measure the frequency of vibration. The frequency is then used to calculate any number of the structural member's physical properties, typically by applying a series of calibration factors.
In conventional systems, the frequency of the vibration is calculated by measuring the average period of the vibration based on the number of zero-crossings, or number of times the wave-form crosses the point of zero amplitude, in a specified time period. While these methods are computationally efficient, they are often subject to error because the methods are unable to distinguish between wire resonance and external noise sources. Particularly, noise sources with a small amplitude and a different frequency than the vibrating wire can introduce substantial errors.
Thus, a new approach is needed that is able to determine the resonant frequency of vibrating wire sensors with improved rejection of external noise sources and improved precision.